1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image used in electrophotography and a process for producing the same.
2. Description of the Related Art
Various processes have been known as electrophotography as described, for example, in U.S. Pat. No. 2,297,691 and JP-B-42-23910 (U.S. Pat. No. 3,666,363). In general, it contains basic process steps, i.e., an exposing step for forming an electrostatic latent image in various methods on a photoreceptor layer utilizing a photoconductive substance, a step of developing the electrostatic latent image with a developer containing a toner, a step of transferring the toner to a recording material, such as paper, a step of fixing the toner image to the recording material by heat, pressure and the like, and a step of removing the toner remaining on the photoreceptor layer.
As the developer used herein, a two-component developer containing a toner and a carrier and a one-component developer using solely a magnetic toner or a non-magnetic toner have been known. A kneading and pulverizing method is generally employed for producing a toner, in which a thermoplastic resin is fused and kneaded with a colorant, a charge controlling agent and a releasing agent, such as wax, and after cooling, the mixture is finely pulverized and classified. Inorganic or organic fine particles are sometimes added to the toner for improving the fluidity and cleaning property thereof. The addition of the fine particles brings about the following problems although an excellent toner can be produced thereby.
In the general kneading and pulverizing method, the toner has an irregular shape and surface structure, which is delicately changed depending on the pulverization property of the material used and the conditions on the pulverization step, and it is thus difficult to control the shape and the surface structure of the toner. Furthermore, there is restriction in selection range of the material in the kneading and pulverizing method. Specifically, it is necessary that the fused and kneaded product is sufficiently brittle before pulverization and can be easily pulverized finely in a production apparatus that can be employed from the economical standpoint. However, in the case where the fused and kneaded product is made brittle to satisfy the requirement, there are some cases where the toner further forms fine powder and causes change in shape due to a mechanical shearing force applied to the toner in a developing device. The phenomena causes such problems that in the two-component developer, the fine powder is stuck on the surface of the carrier to accelerate charge deterioration of the developer, and in the one-component developer, the particle size distribution is broadened to cause scattering of toner and change in toner shape, which brings about poor developing property, whereby the image quality thus obtained is deteriorated.
In the case where a releasing agent, such as wax, is internally added to the toner produced by the pulverizing method, the releasing agent is often restricted to be exposed to the toner surface depending on the combination with the thermoplastic resin. In particular, in a combination of such a resin that is somewhat difficult to be pulverized due to increased elasticity thereof with a high molecular weight component added to the toner, with brittle wax, such as polyethylene, exposure of a large amount of polyethylene on the toner surface is observed. This is advantageous in releasing property upon fixing and in cleaning of an untransferred toner on the photoreceptor, but the polyethylene on the toner surface is easily transferred to the surfaces of the developing roll, the photoreceptor and the carrier by a mechanical force to contaminate them, whereby the reliability is lowered.
The toner sometimes fails to ensure fluidity due to the irregular toner shape even when a fluidity assistant is added thereto, whereby fine particles on the toner surface migrate to depressed parts with a mechanical shearing force in the machine to lower the fluidity of the toner with the lapse of time, and the fluidity assistant is buried into the toner to deteriorate developing property, transfer property and cleaning property. In the case where the toner recovered in the cleaning step is returned to the developing device for reusing, the image quality is further deteriorated. The addition amount of the fluidity assistant is increased to prevent the problems from occurring, such problems occur that black spots are formed, and the fluidity assistant fine particles are scattered.
In view of the circumstances, various production methods of a toner using polymerization, which are different from the kneading and pulverizing method, have been considered. For example, a production method of a toner by suspension polymerization is described, for example, in JP-A-62-73276 and JP-A-5-027476. However, in the case where a toner is prepared in these methods, the controllability of particle size distribution of the toner is substantially equivalent to that of the kneading and pulverizing method, and a classification operation is further necessary in many cases. Furthermore, a toner obtained in these methods has a substantially true spherical shape, which causes considerable deterioration in cleaning property of the toner remaining on a photoreceptor or the like to cause problem in reliability in image quality.
In recent years, a production process of a toner by an emulsion polymerization aggregation method is proposed in JP-A-63-282752 and JP-A-6-250439 as a method for actively controlling the shape and the surface structure of the toner. In the process, a resin dispersion liquid is prepared by an emulsion polymerization method, and a colorant dispersion liquid is separately prepared by dispersing a colorant in a solvent. The dispersion liquids are mixed to form aggregated-particles having a particle diameter corresponding to the particle diameter of the toner, which are then fused and integrated by heating. The process can control the shape to some extent and can improve the charging property and the durability.
There are increasing requirements for a toner used in an electrophotographic process, which is required to have a long service life, a small size, a high processing speed and color reproduction. In particular, a fixing device having high speed operation and small size requires such a toner that is fused at a high temperature in a short period of time, causes no offset contaminating an image, and exhibits stable fixing property on the position of the fixing device and fluctuation in temperature with the lapse of time. In the emulsion polymerization agglomeration method, particularly, further optimization of the composition has been carried out to satisfy the requirements. It is possible that the aggregation degree of the binder resin is increased by increasing the amount of the aggregation agent, so as to prevent offset. However, the acceleration of aggregation increases the amount of coarse particles contained in the toner to cause such problems that the uniformity of the images is deteriorated, and the yield of the toner is significantly lowered. It is also possible that hot offset of the toner is controlled by increasing the amount of the releasing agent but not changing the amount of the aggregating agent, but the amount of wax contained in the toner is increased to lower dispersion property of carbon black in the case of a black toner, whereby the charging property is lowered, and the dielectric loss is increased. As a result, the developing property and the transfer property are deteriorated, and thus, sufficient performance cannot be obtained. In the case where a color image is formed, sufficient coloring property cannot be obtained due to aggregation of the colorants. It is also resulted that the growth of the particle diameter on the particle forming step is lowered, and as a result of increase of the processing temperature, the amount of coarse particles in the toner is increased to cause deterioration in uniformity in images. Furthermore, there is another problem that the image strength (crease) after fixing is deteriorated.
In order to prevent high temperature hot offset from occurring, such a toner for developing an electrostatic image has been proposed that has a melting heat, a glass transition temperature and a melting heat amount satisfying a specific relational equation (see JP-A-2001-117273).